MIPS objcopy --rename-section fix

[Maciej W. Rozycki]

On Tue, 12 Jul 2016, Alan Modra wrote:

> >  What is wrong with `tx39-*-*' specifically?  I've had `tx39-elf' included 
> > in my routine testing and I haven't seen any issues beyond those common to 
> > all MIPS/ELF targets using IRIX-style ELF structures.
> 
> tx39 is no longer the canonical machine name.  It's fine to use
> "tx39-elf" when configuring, but internally we now use mipstx39.

 Indeed, as a look at `config.sub' reveals, and no longer as in: since 
time immemorial. ;)  Thanks for the hint!

  Maciej